Plating high strength steel without hydrogen embrittlement

ABSTRACT

A PROCEDURE IS PROVIDED FOR THE TREATMENT OF HIGH STRENGTH STEELS SO THAT THEY MAY LATER BE ELECTROPLATED IN CERTAIN STABLE CYANIDE-ZINC AND STABLE CYANIDE-CADMIUM PLATING BATHS OF PRECISE COMPOSITION. THE PROCEDURE INVOLVES FIRSTLY CLEANING AWAY VISIBLE FILMS FROM THE STEEL SURFACE, E.G. BY USING HYDROCHLORIC ACID OR SULFURIC ACID. THE SECOND STEP INVOLVES PRODUCING A THIN FUGITIVE POROUS METAL FILM WHICH IS CATHODIC TO THE STEEL ON THE STEEL SURFACE, E.G. BY IMMERSING THE STEEL IN A BATH OF COPPER SULPHATE TO PRODUCE SUCH A COPPER FILM. THE THIRD STEP INVOLVES REMOVING THE METAL COATING AD SIMULTANEOUSLY FORMING A SMOOTH SURFACE FREE OF MINUTE SHARP EDGES, E.G. BY TREATMENT WITH AN AQUEOUS NITRIC ACID-ACETIC ACID CLEANING SOLUTION OR A AN AQUEOUS NITRIC ACID-ACETIC ACIDPHOSPHORIC ACID CLEANING SOLUTION. THE FINAL STEP INVOLVES REMOVING SMUT FROM THE STEEL SURFACE, E.G. BY WASHING WITH WATER UNDER HYDRAULIC PRESSURE, WITH ADDED ULTRASONIC VIBRATION OR WATER UNDER PNEUMATIC PRESSURE. THE PROCEDURE ALSO INCLUDES USING A STABLE CYANIDEZINC BATH WHOSE COMPOSITION STAISFIES THE FORMULA (OH-NORMALITY)-(CN-NORMALITY)=1,2 AND THE RATIO OF CN-NORMALITY/ZN++ NORMALITY RANGES FROM 1.07 TO 1.25; AND THE USE OF A STABLE CYANIDE-CADMIUM BATH WHOSE COMPOSITION SATISFIES THE FORMULA (OH-NORMALITY)=(CN-NORMALITY) AND THE RATIO OF CN-NORMALITY/CD++NORMALITY IS ABOUT 2.8.

United States Patent 3,647,648 PLATING HIGH STRENGTH STEEL WITHOUT HYDROGEN EMBRITTLEMENT Wilfred Dingley, Ottawa, Ontario, John S. Bednar, Gatineau Pointe, Quebec, and Raymond R. Rogers, Ottawa, Ontario, Canada, assignors to Canadian Patents and Development Limited, Ottawa, Ontario, Canada No Drawing. Filed May 18, 1967, Ser. No. 639,312

Int. Cl. C23b 1/00 US. Cl. 204-34 19 Claims ABSTRACT OF THE DISCLOSURE A procedure is provided for the treatment of high strength steels so that they may later be electroplated in certain stable cyanide-zinc and stable cyanide-cadmium plating baths of precise composition. The procedure involves firstly cleaning away visible films from the steel surface, e.g. by using hydrochloric acid or sulfuric acid. The second step involves producing a thin fugitive porous metal film which is cathodic to the steel on the steel surface, e.g. by immersing the steel in a bath of copper sulphate to produce such a copper film. The third step involves removing the metal coating and simultaneously forming a smooth surface free of minute sharp edges, e.g. by treatment with an aqueous nitric acid-acetic acid cleaning solution or an aqueous nitric acid-acetic acidphosphoric acid cleaning solution. The final step involves removing smut from the steel surface, e.g. by washing with water under hydraulic pressure, with added ultrasonic vibration or water under pneumatic pressure.

The procedure also includes using a stable cyanidezinc bath whose composition satisfies the formula (OH' normality)(CN normality)=1.2 and the ratio of CN- normality/Zn++ normality ranges from 1.07 to 1.25; and the use of a stable cyanide-cadmium bath whose composition satisfies the formula (OH normality)=(CN normality) and the ratio of CN" normality/Cd normality is about 2.8.

This invention relates to the minimization of hydrogen embrittlement particularly in the plating of high strength steels. It relates both to the treatment of previously hydrogen embrittled high strength steels to remove or at least substantially to reduce the embrittlement, and to the plating of high strength steels by a technique which substantially avoids or minimizes the formation of hydrogen embrittlement of the high strength steel.

It is known that high strength steels become seriously embrittled during surface pretreatment, particularly when they are electroplated with metals such as zinc and cadmium. The effect of hydrogen embrittlement is the reduction of ductility so that the metal tends to fail by brittle fracture rather than by plastic deformation.

It appears that there are no ferritic steels which are immune to hydrogen embrittlement, but there are rather wide variations in the degree of susceptibility of the many types of steels that fall in this category. In a general way, these variations can be correlated with the composition and micro structure. The embrittlement occurs at all strength levels but it usually is most severe at what is termed ultra high strength levels, namely 200,000 p.s.i. and above.

In other words, the problem of hydrogen embrittlement of steel parts which are finished by electroplating becomes acute when medium and high strength steels must be processed. Generally, steels having an ultimate tensile strength of about 150,000 p.s.i. or less are not significantly affected by the hydrogen to which they are exposed during finishing; the steels with strengths in the ice neighbourhood of about 180,000 to 220,000 p.s.i. are adversely affected and are subject to premature failure. Those steels in the range of 260,000 to 280,000 p.s.i. are usually so susceptible to hydrogen as to be unsafe for structural use unless special precautions are taken to relieve or prevent their embrittlement.

For structural uses, particularly in aircraft, rusting is intolerable and it must be prevented by suitable finishes. The preferred methods of finishing utilize sacrificial coatings so that local corrosion does not occur at spots where the protective coatings may have become chipped or marred. The sacrificial coating of choice is cadmium but it must be deposited or handled in such a way as to ensure non-embrittlement of the coated parts.

A serious difficulty arises in this connection because there is no positive non-destructive test for determining hydrogen embrittlement. A high strength plated part may appear to be perfect and until it fails under sustained load at some indefinite time after installation, there is no indication that it has been embrittled during processing or finishing. This circumstance leads to another serious situation, namely lack of dependability and for this reason the use of high strength steels has been limited, particularly in air-frames. This limitation in turn imposes weight penalties and in the case of structures in which each ounce is important, all these factors provide reasons for avoiding or minimizing such hydrogen embrittlement.

There have been many suggestions for solving the embrittlement problem in steel. One practical way to remove hydrogen embrittlement is baking. Baking is a diffusion process and as such is controlled by time and temperature. Because the rate of diffusion increases exponentially with increase in temperature, it is always desirable to bake at the highest possible temperature thereby reducing markedly the time required. Production costs will also be lowered because oven time will be decreased. Baking is thus an added expense, and moreover, it is not always effective. Other metallurgical considerations must always be taken into account, however, in choosing a baking temperature. Almost all alloys which are likely to be embrittled are either heat-treated or cold-worked to produce certain properties. Obviously a baking temperature high enough to remove even partially the effects of such treatment must be avoided.

Avoidance of hydrogen embrittlement in the first place is, of course, the most desirable state of affairs. Two avenues of approach are open, namely employing those processes which do not produce hydrogen embrittlement or secondly, applying a barrier coating to the basic metal prior to the operation which normally produces embrittlement.

Some of the techniques which have been suggested for solving the embrittlement problem include the use of thin coatings deposited at high current density. Such coatings are porous and any embrittlement encountered may usually be relieved by baking after plating. This method, However, has the disadvantage of providing inferior protection against corrosion and the added disadvantage that baking may not entirely relieve embrittlement.

Plating of cadmium from organic electrolytes is another way to prevent hydrogen deposition as a metal is applied. Not only do the organic electrolytes have relatively poor conductivity, but usually, amongst other disadvantages which interefere with the use of such process, the deposits are also inferior in quality. Furthermore, personnel may develop allergies to the materials used in the baths.

One hundred percent cathodically efficient aqueous electrolytes which could be used to deposit cadmium without depositing hydrogen have also been suggested. It has been suggested that electrolytes, such as cadmium fluoborate or high cadmium-low cyanide baths, may be used. However, these. solutions require very careful control and there is little or no margin of safety in their operation.

Another technique suggested is the suppression of hydrogen formation in an aqueous solution for cadmium plating. This has been proposed by using a controlled sodium-cadmium-cyanide bath.

Another approach suggested was the application of a barrier coating to prevent contact of the hydrogen with the steel surface. However, this has not been found to be practical.

A further technique suggested has been the use of vacuum evaporated coatings which are not embrittling. At present these are very expensive and cannot be used for large parts in currently available apparatus.

Another procedure which has been suggested is a multiple step plating, but it has been found that this is not dependable from a processing standpoint since it has not uniformly prevented embrittlement.

Still another procedure which has been proposed in avoiding embrittlement was the use of a very efiicient sodium-cadmium-cyanide plating bath with the addition of sodium nitrate to suppress gassing. The use of nitrate in a cadmium plating solution has traditionally been avoided because, aside from depolarizing the cathode, it had an adverse effect on the efficiency of the solution. Also, classically, nitrates were seldom used in plating baths because they may be cathodically reduced to ammonia. However, in order to overcome the loss of efficiency in throwing power caused by the nitrate, the inclusion of a proper throwing power additive was necessary.

It has also been suggested that the high strength steels should be cleaned in an exacting manner prior to electrodeposition. It has been found that conventional cleaning processes which involve the use of strong acids or alkali's cannot be used with the susceptible steels unless the hydrogen which they introduce is subsequently baked out. Because baked surfaces are oxidized and are unfit for plating, it was found to be necessary to use other pre-plate cleaning methods. For medium and high strength steel articles, it has been suggested that baking may be used to relieve any imbrittlement before pre-plating cleaning is started. Furthermore, as the surfaces to be plated must be free of contamination and must not be processed in any manner likely to cause hydrogen embrittlement, the treatment of the steel becomes critical. The medium strength steels were therefore proposed to be handled as little as possible. It was suggested that they be dry sandv blasted, air blown or rinsed and then plated immediately. Particularly careful handling of, the parts after sand blasting to avoid fingerprints or any other surface contamination was suggested as being essential.

For high strength steel parts, anodic alkaline cleaning followed immediately by a water rinse was recommended, after the preliminary baking. However, it was found that enough embrittlement normally occurred in an alkaline cleaning bath without the anodic current to preclude its use on high strength steels. The parts, therefore, must be kept anodic in the bath and must be rinsed immediately after being cleaned. It was also suggested that, if there was any visible surface contamination after this step, the parts be air dried and then be cleaned further by blasting with clean dry abrasive and again water rinsed. Itwas also suggested that they next be electrohoned by the use of a sulphuric-phosphoric acid bath and immediately water rinsed. It was then taught that the parts be immersed in a plating solution for 15 minutes without current and then finally be electroplated in the same solution.

The importance of having the high strength parts absolultely clean before plating cannot be overemphasized. It is observed that even a thin film left on the steel by handling the parts without gloves may cause marked embrittlement. Also the parts must be thoroughly rinsed immediately after the anodic alkaline cleaning and the electrohoning as it was found that residual alkali or acid on the surface can produce embrittlement.

An object of one aspect of the present invention is the provision of a pre-cleaning treatment of steel by which the steel is rendered more suitable for subsequent zinc or cadmium electro-deposition.

An object of another aspect of the present invention 1s the provision of a procedure for removing hydrogen embrittlement near the surface of steel parts which have bene hydrogn embrittled by various conventional treatments.

An object of a further aspect of the present invent on is the provision of a process for providing a corrosion preventive plating on a high strength steel 1n such a manner as to minimize the hydrogen embrittlement 1n the ste l.

in object of a specific aspect of the present invention is the provision of a process for Zll'lC electro-depos tion on steel without producing hydrogen embrittlement in steel.

An object of another specific aspect of the present invention is the provision of a procedure for cadm um electro-deposition without producing hydrogen embrittlement in steel.

By a broad aspect of the present invention a process is provided for pre-treating a high strength steel surface prior to electro-depositing a corrosion protective metal thereon, to provide a clean, smooth, substantially ox de free surface conducive to electroplating without causing significant hydrogen embrittlement which comprises effecting one of the following procedures, namely: procedure (A) comprising providing a surface of the steel smooth, clean and substantially oxide free; then treatlng said surface with an acid solution comprising nitric acidacetic acid-phosphoric acid to maintain said surface smooth, clean and substantially free of oxide, and finally removing smut from the steel surface after the acid treatment; and procedure (B) comprising cleaning away visible oxide film from the steel surface, then producing a thin fugitive porous film of copper, or zinc, then removing the metal film while simultaneously produclng a smooth surface substantially free of minute sharp edges by treatment with an acid solution comprising nitric acid and acetic acid, and finally, removing smut from the steel surface after the acid treatment.

By a preferred embodiment of this aspect of the present invention, such a process is provided which compnses: (a) cleaning away visible oxide film from a steel surface with hydrochloric acid; (b) depositing electrolessly on the cleaned steel surface, a thin fugitive porous copper flash; (c) contacting the deposited copper coating with an acid cleaning solution containing nitric acid and acetic acid; and (d) removing smut and cleaning the surface produced by step (c) by contacting the surface with water in conjunction with ultrasonics, or under pressure, eg.. hydraulic pressure or pneumatic pressure.

By a preferred embodiment of this aspect of the present invention, the acid solution used in step (c) comprises an aqueous solution of nitric acid and acetic acid.

According to another aspect of the present invention, a process is provided for plating high strength steel with a corrosion protection metal coating which process comprises the steps of: (I) pre-treating said high strength steel to provide a clean, smooth, substantially oxide free surface conducive to electro-plating without causing significant hydrogen embrittlement which comprises effecting one of the following procedures, namely: procedure (A), comprising, providing a surface of the steel smooth, clean and substantially oxide free; then treating said surface with an acid solution comprising nitric acid-acetic acid-phosphoric acid to maintain said surface smooth, clean and substantially free of oxide, and finally removing smut from the steel surface after the acid treatment; and proceure (B), comprising, cleaning away visible oxide film from the steel surface, then Producing a thin, fugitive porous film of copper, or zinc,

then removing the metal film while simultaneously producing a smooth surface substantially free of minute sharp edges by treatment with an acid solution comprising nitric acid and acetic acid, and finally, removing smut from the steel surface after the acid treatment; and electroplating said corrosion protective metal coating on said pretreated steel from a stable cyanide bath of the ions of said protective metal.

By one embodiment of this aspect of the present invention, the metal being electroplated is zinc and the stable cyanide bath is a stable cyanide bath of zinc ions.

By another embodiment of this invention, the metal being electroplated is cadmium and consequently the metal ions in the stable cyanide bath are cadmium ions.

In either of these two embodiments the acid used in step (c) may comprise an aqueous solution of nitric acid and acetic acid. Alternatively, in either of these two embodiments, the acid solution used in step (c) may comprise an aqueous solution of nitric acid, acetic acid and phosphoric acid.

By another embodiment of this invention, the plating bath is a stable cyanide-zinc bath whose composition satisfies the formula (OH- normality)-(CN normality)=1.2. Preferably, also, the bath further is one in which the ratio of CN'- normality/Zn normality ranges from 1.07-1.25.

In a particularly preferred embodiment, the bath has the following analysis, namely NaCN, 89 g./1.; NaOH, 117 g./l.; and Zn, 40 g./l.

In another particularly preferred embodiment, the bath has the following analysis, namely, NaCN, 44 g./l.; NaOH, 88 g./l.; and Zn, 23 g./l.

By yet another particularly preferred embodiment, the bath has the following analysis, NaCN, 71 g./l.; NaOH, 107 g./l. and Zn, 31 g./l.

By another embodiment of this aspect of the present invention, the plating bath is a stable cyanide-cadmium bath in which the (OI-l" normality)=(CN normality). Preferably, also, the bath further is one in which the ratio of CN' normality/Cd++ normality is about 2.8.

In a particularly preferred embodiment of this aspect of this invention, the bath has the following analysis, namely: NaCN, 57.8 g./1.; NaOH, 44.3 g./l.; and Cd, 21.0 g./l.

In another particularly preferred embodiment, the bath has the following analysis, namely: NaCN, 98.3 g./l.; NaOH, 80.3 g./l.; and Cd, 34.5 g./l.

In still another particularly preferred embodiment, the bath has the following analysis, namely: NaCN, 137.3 g./l.; NaOH, 112.5 g./l.; and Cd, 48.8 g./l.

As pointed out hereinabove, according to one aspect of this invention, it is necessary to carry out four functional operations on the steel in order to make it suitable for electroplating with a protective metal and still inhibit or prevent the prevalence of hydrogen embrittlement.

These four steps are as follows:

Firstly, cleaning away visible films of foreign materials, e.g. oxide films, from the steel surface. In accordance with one aspect of the present invention, it was found that hydrochloric acid was particularly successful in removing oxides while keeping the metal dissolution at a low rate. Another acid which may be used to remove the visible film of foreign materials is sulfuric acid, although this is not the acid of choice, since it tends to increase the smut problem. In addition, it has been found that the concentration of the hydrochloric acid is not critical, for it was found that any percentage concentration of hydrochloric acid would clean the steel surface. However, it has been found that a concentration of hydrochloric acid of 18% by weight is optimum for cleaning within 13 minutes in order to minimize hydrogen embrittlement.

Secondly, the steel must then be treated in order to produce or maintain a clean, smooth substantially oxide free surface conducive to electroplating without causing significant hydrogen embrittlement. One such procedure involves producing a very thin porous coating of a fugitive metal coating. Metals which may be used include copper and zinc, but copper is the preferred such metal to use in forming the fugitive metal coating. This may be achieved by treatment in a copper sulphate solution by a straight replacement of iron by copper. The concentration of copper sulphate in the aqueous solution is not critical since if any copper ions are in solution they will be replaced by iron ions. However, the pH of the solution is important, and an acid pH of 2.2-2.8 is necessary for operability. A pH of 2.6 is preferred. The formation of this fugitive metal, e.g. copper coating, assists in producing and maintaining a smooth steel surface during the subsequent acid treatments of the steels.

Thirdly, treatment with an acid cleaning solution including nitric acid and acetic acid produces and/or maintains a smooth surface free from minute sharp edges Without materially increasing the temperature of the acid solution. One particular acid solution which also simultaneously removes the copper or zinc coating is a solution having a ratio of from 10-15 percent by Weight nitric acid, and from 4 to about 35 percent by weight acetic acid with the balance being water. Another such acid solution which has been particularly successful is an aqueous solution containing 23.3 percent by weight of nitric acid, from 31.1 to 34.3, preferably 32.7, percent by weight of acetic acid, and from 27.9 to 29.7, preferably 28.3, percent by weight of phosphoric acid, the balance being water which was contained in the nitric, acetic and phosphoric acids.

Fourthly, subsequent treatment to remove the thick smut from the steel surface after the treatment with the nitric-acetic acid solution. One particular procedure which applicant has found to be most desirable is rinsing with water. The water may be water under hydraulic pressure or water under pneumatic pressure. Another procedure which is useful is ultrasonic vibration in conjunction with the water rinsing. Still another procedure which applicant has found to be successful is air-water blasting which is almost as effective and much less expensive than ultrasonic treatment.

It has been found, according to another aspect of this invention, that it is essential to use a stable cyanidemetal ion bath for the purpose of plating after the steels have been pre-treated to remove hydrogen embrittlement from near the surface of the steel and to minimize re-formation of hydrogen embrittlernent. Applicant has found that such baths which may be used for the electroplating of zinc are purified plating baths whose proportions of ingredients satisfy the formula (OH"- normality) (CN* normality) =12 Preferably, the proportions of ingredients also satisfy the condition that the ratio of CN- normality/Zn++ normality=1.2

For the purpose of electroplating cadmium onto the pre-treated steel surface, it has been found that cadmiumcyanide baths useful in this invention are those in which the OH- normality is equal to the CN- normality. Preferably, also, the bath further is one in which the ratio of CN normality/Cd normality is about 2.8.

The following disclosure provides descriptions and results of tests which have been conducted in order to evaluate the present invention in its various aspects. It also provides examples of the utility of various aspects of the present invention.

The steels, whose analyses are tabulated below, were tested according to the procedures of aspects of the present invention: high strength steels AISI Type 1062, AISI Type 4037; AISI Type E4340; and mild steel AISI Type 1010.

TEST MATERIALS AND EQUIPMENT The steels of Types 1062 and 4037 tested according to aspect of this invention had been supplied in the form of pins, and the degree to which each of these was embrittled during a test was expressed in terms of the angle of bend required to produce rupture. On the other hand, the Type E4340 steel had been supplied in the form of rods, and the degree of embrittlement of this material was expressed in terms of the length of time required to produce rupture in a specially prepared bar of the notch stretch rupture type. The bars were stretched by a load calculated on the basis of 75% of the ultimate tensile strength of the steel, the latter value having been determined in the machined and heat-treated condition prior to surface pretreatment and plating.

The pins of Type 1062 steel were 3.25 in. in length and 0.15 in. in diameter. They had been drawn from wire and austempered in the bainitic range, resulting in a Rockwell C hardness of 52-56 and an ultimate strength of 257-279 K. p.s.i. The material was covered with a uniform thin scale of 'blue iron oxide.

The pins of Type 4037 steel were 2.63 in. in length and 0.15 in .in diameter. They had been drawn from wire and then quenched and tempered, resulting in a Rockwell C hardness of 51-55. They were covered with a thin, very porous film of copper which had been applied to prevent decarburization during heat treatment, together with a certain amount of smutty, black material.

The test bars of Type E4340 (rough machined) steel had been hardened in a neutral salt bath at 1500i25 F. for 30 min. and quenched in oil maintained at 75- 140 F. They then were tempered at 400i10 F. for 4 hr. and air-cooled, resulting in a Rockwell C hardness of 50-54. After finish machining, the bars were stress-relieved at 375:L-25 F. for 4 hr. The 60 notch with a radius of 0.005- ';0.00l in. then was machine-cut into the test bar, producing a diameter of 0.17681-0000-1 in. The ultimate strength of five test bars, chosen at random, ranged between 323 and 340K p.s.i. with an average of 332K p.s.i. The metal at the notch was free from any visible film of oxide or other non-metallic material.

TEST EQUIPMENT The equipment used in testing products produced according to one aspect of this invention was as follows:

Sonogen ultrasonic generator Model LG-l50, 25 kc. 150 w, equipped with an LT-60 transducerized tank. Branson Ultrasonic Corp.;'

Daniels motor-driven portable plating barrel Model 35, 6 r.p.m. volume of plating solution used, 8 litres);

Hounsfield notched bar bending jig attached to a tensometer machine Type K. During testing, the unnotched pin is supported at two points 1.19 in. apart. Pressure applied to a 0.125 in. diameter mandrel forces the pin into the gap between these two points until it breaks or until a bend of 90 has been produced at the end of 7 min.

Satec creep and stress rupture machine, special Model C, max. capacity 12,000 lb.; Lorco Liquamatte wet-blasting equipment Model 22. Line pressure 80 lb./in.

A first series of tests was conducted to develop the procedures, according to one aspect of this invention, to pretreat the steel before the electroplating procedures. These tests were carried out in the following manner.

Between 12 and 40 pins of Type 1062 or 4037 steel, and 1 or 2 bars of Type E4340 steel, were treated in each surface pretreatment procedure. These procedures were performed at room temperature in 600-ml. glass beakers containing 300-400 ml. of aqueous solution. The con- Nitric acid (HNO 15 12 A B Acetic acid (CH CO-OH) 4 34 Copper sulphate solution for copper immersion coating (CuSO .5H O)-10% (pH- -2.6).

City tap water was used in the preparation of these solutions as well as in all rinsing.

When working with Type 1062 and 4037 steels, each surface pretreatment procedure tested was evaluated with regard to the cleanliness and smoothness of the pins produced using scales from excellent to poor, and with regard to the degree of embrittlement of the pins as indicated by the angle of bend at which they were fractured. When working with Type E4340 steel, the test bars were evaluated with regard to embrittlement only. The term excellent cleanliness was used in cases where the steel surface was very active as shown by the fact that rapid, uniform rusting appeared immediately after the surface was exposed to the atmosphere.

The term excellent smoothness was used in cases where the steel surface appeared to be free from the minute, sharp edges which can be seen on many steel surfaces after treatment when inspected at a magnification of X.

RESULTS OF PROCEDURES TESTED Theresults of the first procedures tested are summarized below in Table II.

TABLE II [Surface conditions and degree of embrittlement of Type 1062 and 4037 steel pins alter pretreatments] Degree of embrittlement Surface condition Angle of Proportion of Steel bend at pins fractured Test No. Cleaning treatment type Cleanliness smoothness fracture (percent) 6 None "1 45-115 33 7 H2504 3 min), rinse ag 13g 8 nzsoi (a min.) with ultrasonics, rinse- 3533 1% 9 H01 (3 min.), rinse S5353 g2 10 HCl (3 min.) with ultrasonics, rinse gg 11 H01 (1 min.) with ultrasonics, rinse gigs 123 In the first series of tests, the effects of surface pretreatments involving sulphuric acid with and without ultrasonics, and hydrochloric acid with and without ultrasonics on Types 1062 and 4037 steels were investigated. The results of Tests 6-11, presented in Table II, show that the Type 1062 steel: (a) was embrittled during all of the procedures used except those involving hydrochloric acid with ultrasonics, the l-min. procedure of this latter type giving as good results as the 3-min. one; (b) was not greatly improved in cleanliness by any of the procedures except those involving hydrochloric acid and even these did not give excellent results; became less smooth in all of the procedures. They also show that the Type 4037 steel was very little improved with regard to cleanliness and was not improved at all with regard to smoothness by these various procedures. In every case, the embrittlement due to the pretreatment described was much greater than desired.

Accordingly, the procedures summarized above in Table II are outside the ambit of the pretreating procedure of one aspect of this invention.

Since the procedure involving a 1-min. treatment in hydrochloric acid with ultrasonics proved to be the most desirable in the first series of tests, certain modifications of this procedure were investigated in Tests l2l6, the summarized results of which are presented in Table III.

TABLE III Surface conditions of Type 1062 and 4037 steels after pretreatments (the angle of bend of 135 was reached without fracture in all cases) Surface condition after 14.-.: H01 (1 min.) with ultrasonics, rinse, HNO QH3COOH(A) (3 min.), r nse, ultrasonic water rinse.

15.... B01 (1 min.) with ultrasonics rinse, copper coat, rinse, iIN 03-on3ooo11 (A) (1 min.), rinse, ultrasonic water rinse, H01 (1 min.) with ultrasonics, rinse.

16.... H01 (1 min.) with ultrasonics rinse, copper coat, rinse, HNO3CH3COOH (A) (1 min.), rinse, airwater blast.

do Excellent. 4037 Do.

From the above table, it is seen that a nitric acid treatment, after a hydrochloric acid treatment similar to that used in Test II, did not change the degree of cleanliness of the Type 1062 steel but decreased its smoothness. On the other hand, it did improve the cleanliness of the Type 4037 steel while decreasing its smoothness.

It is seen in Test 13, that the addition of copper sulphate (copper coat), nitric acid, and hydrochloric acid with ultrasonics treatments after hydrochloric acid treatment similar to that used in Test 11 made a definite improvement in the cleanliness of both steels while decreasing their smoothness. It was shown in Test 14 that the use of the HNO CH COOH (A) cleaning solution treatment instead of the nitric acid treatment used in Test 12 produced excellent smoothness and surface cleanliness on the Type 1062 steel, and excellent surface cleanliness and only good smoothness on the Type 4037 steel. It should be noted that surfaces having excellent smoothness sometimes had very small, shallow indentations. However, these had none of the sharp edges that are particularly undesirable in plating.

It is seen in Test 15 that the addition of a copper coat treatment before the HNO CH COOH (A) cleaning solution used in Test 14, and of a final hydrochloric acid treatment, produced excellent smoothness and cleanliness in both types of steel. It is seen inTest 16 that an airwater blast treatment may be used to replace the last two ultrasonic treatments used in Test 15. While the results of Test 16 showed that treatment with hydrochloric acid and ultrasonics after the nitric acid-acetic acid treatment gave an excellent surface, it is preferred not to do this additional step. It is believed that such additional step introduces the possibility of reintroducing hydrogen embrittlement.

As a result of the tests hereinabove described, it has been found that Types 1062 and 4037 high strength steels can be treated in accordance with one embodiment of the present invention without significant embrittlement when the following requirements are met:

I. The steel surface is made clean, smooth, and free from embrittlement by the procedure: HCl (10 sec.), rinse, copper coat, rinse, HNO CH COOH (B) (5 min.), rinse, ultrasonic water rinse. It was also found that the usefulness of Type E4340 high strength steel was greatly increased after treatment by this same procedure.

It is therefore noted that the most important steps in the pretreatment according to one aspect of the present invention may be summarized as follows:

(A) Cleaning away visible films of foreign materials from the steel surface. Hydrochloric acid was found to be particularly successful in removing oxides while keeping the metal dissolution at a low rate.

(B) Treatment in copper sulphate solution to produce a very thin copper coating on the steel. This assists in producing and maintaining a smooth steel surface during the subsequent nitric-acetic acid cleaning solution treatment of Type 4037 and E4340 steels in particular.

(C) Treatment with an acid cleaning solution containing nitric acid and acetic acid, e.g. an aqueous nitric acidacetic acid solution or an aqueous nitric acid-acetic acidphosphoric acid solution which produces a smooth surface free from minute sharp edges, without materially increasing the temperature of the acid solution. It also removes the thin copper coating.

A a novel technique has also been provided in order to prolong the length of time that the nitric acid-acetic acid cleaning solution can be used. This novel technique is as follows:

(1) When starting with a freshly prepared nitric acidacetic acid or nitric acid-acetic acid-phosphoric acid solutions, steel specimens to be treated are coated with copper from a copper sulphate solution as described hereinabove. Some of this copper will dissolve in the nitric acidacetic acid or nitric acid-acetic acid-phosphoric acid solutions and thus the copper content of the solutions will continue to increase.

(2) To avoid an excess of copper in the solutions, some batches of steel specimens should be placed into the nitric acid-acetic acid or the nitric acid-acetic acid-phosphoric acid solutions without being copper coated in the copper sulphate solution but with the surface being wet with water. Copper present in the nitric acid-acetic acid or in the nitric acid-acetic acid-phosphoric acid solutions then will be deposited on the specimens by replacement and thus form the desired copper coating for the treatment.

(3) When it becomes difiicult to obtain a copper deposit from the nitric acid-acetic acid or the nitric acid-acetic acid-phosphoric acid solutions because of low copper content, then the steel specimens should be copper coated in the copper sulphate solution. This will increase the copper content of the nitric acid-acetic acid or nitric acidacetic acid-phosphoric acid solutions once again.

This procedure can be repeated numerous times before the nitric acid-acetic acid or the nitric acid-acetic acidphosphoric acid solutions cease to clean the steel surfaces satisfactorily.

(4) Use of ultrasonic vibration in conjunction with acid treatment and water rinsing. This greatly increases 1 I the efiiciency of these processes. It was shown that air-water blasting to remove thick smut from the steel surface after acid treatment is almost as effective and much less expensive than ultrasonic treatment.

By another aspect of this invention, the steels pretreated according to the first aspect of this invention are electroplated using a particular type of plating bath. The following tests show the development of (A) zinc electroplating baths, and (B) cadmium electroplating baths.

(A) ZINC ELECTROPLATING BATHS Steel pins The steel pins used for these particular tests had been produced from drawn steel (Type 1062) wire and had been austempered in the bainitic range resulting in a Rockwell C hardness of 52-56. They had been obtained from two different production batches; the surfaces of the pins of one batch were covered with a thin uniform blue coating of iron oxide containing very little carbonaceous smut (later referred to as pins without smut), and the surfaces of those of the second batch were covered with a combination of oxide and a thin extremely adherent deposit of carbonaceous smut (later referred to as pins with smut). The latter was the more normal condition. The smut in this case apparently had been exposed during the pickling of the steel rod and then, during the wiredrawing process, had been pressed into the surface of the finished wire. The pins were 0.15 in. in diameter and varied in length between 2.25 and 3.25 in.

The following equipment was used in carrying out the tests of the procedures of this aspect of this invention:

' (1) A vapor degreaser containing suitably stabilized trichloroethylene;

(2) Glass containers for pickling solution, which consisted of concentrated (37 percent) reagent hydrochloric acid diluted with distilled water to 18 percent by weight;

(3) A glass anodic cleaning cell with a mild steel cathode and an electrolyte containing 10 percent by weight of sodium hydroxide in distilled water. The anode current density was 95 amp./ sq. ft;

(4) A wet-blasting cabinetin which a slurry of Novaculite Type NE of grain size 1250 (a proprietary material marketed by the Wheelabrator Corp.) and water was forced through a 0.25 in. nozzle at 80 lb./ sq. in. line pressure to impinge on the metal surface;

(5) A pickling tank in which two ultrasonic transducers containing Sonogen Z elements (product of the Branson Ultrasonic Corp.) operating at a frequency of 25:3 percent and a load of 135-140 milliamperes, were submerged in water 6 in. apart. Specimens to be pickled were placed in a glass beaker containing 18 percent hydrochloric acid, and this in turn was placed between the transducers, the pickling time being one minute.

Electroplating The electroplating portion of the tests was performed in 2.3 litres of electrolyte in a rectangular glass container 5 in. Wide by 7.5 in. long by 5 in. deep. A zinc anode, cut from an ingot of 99.99 percent purity, was placed against each of the two narrow sides of the container, the immersed portion of each anode being 1.75 by 3.5 in. The cathode was centrally located between the two anodes and immersed at least 0.5 in. below the surface of the electrolyte. A slowly rotating glass propeller slightly agitated the electrolyte which was maintained at 77 :2 F. (25 C.). The direct current electricity was supplied from a copper/ copper oxide rectifier suitably equipped with controls and meters.

The plating solutions used in most of the laboratory tests were prepared from sodium cyanide (NaCN), sodium hydroxide (NaOH) and zinc oxide (ZnO) of each either reagent quality, or commercial plating quality. These chemicals were dissolved either in distilled water or in tap water. The method of purifying the solutions to remove metal and carbonate ions was the zinc dust electrolytic 12 process. This procedure consists of adding zinc dust to the solution, electrolyzing the solution for one hour at 75- amp/sq. ft. and 77 F. (15 C.) and filtering.

The plating baths were frequently filtered through Whatman No. 52 filter papers to remove foreign particles. Distilled water was used to compensate for losses due to evaporation. Each bath was analyzed frequently to detect any changes in composition that might have occurred.

Bend testing A slow bend test was used as a standard performance test during this investigation because pins of this type frequently are bent in service. In this test an electroplated steel pin which had not been post-baked is place in a Hounsfield notched bar bending jig attached to a tensometer machine Type K. The unnotchcd pin is supported at two points located 1.19 in. apart. Pressure applied to a 0.125 in. diameter mandrel forces the pin into the gap between these two points until it breaks or a bend of has been produced at the end of 7 minutes. The pressure required to bend the pin may be read on a gauge attached to the machine. When a pin is to be bent through an angle greater than 90 it is first bent on the tensometer machine and then placed in a vise so that only the ends of the pin are in contact with the jaws of the vise. The vise is gradually closed, thus slowly bending the pin. Any pin remaining unbroken after bending through 90 is considered satisfactory from the standpoint of hydrogen embrittlement.

Coating thickness testing The thicknesses of the coatings produced by zinc electroplating were determined by means of the Aminco- Brenner Magne Gage.

The first tests carried out were slow bend tests on the two types of pins in the as received condition, and also after they had been pickled for one minute in 18 percent hydrochloric acid and rinsed. In an additional experiment the pins with smut were pickled by the same procedure while under the influence of ultrasonic vibrations. The results are shown in Table IV, below:

It is seen from the above Table that both types of pins as received gave satisfactory bend test results, and the same was true of the pins without smut after pickling. Although the pins with smut gave unsatisfactory results when pickled by the ordinary procedure, they gave satisfactory resuits when pickled under the iruluence of ultrasonic vibrations.

The following tests were then carried out:

1) The pins were vapor degreased, pickled for one minute in 18 percent hydrochloric acid, rinsed in tap water and then in distilled water.

(2) The same as in (1) followed by anodic cleaning in 10 percent sodium hydroxide solution at a current density of amp/ sq. ft., rinsing in tap water and then in distilled water.

(3) The pins were vapor degreased, wet-blasted, scrubbed, rinsed in tap water and then in distilled water.

(4) The same as in (3) followed by pickling in 18 percent hydrochloric acid for three minutes, rinsing in tap water and then in distilled water.

The same as in (3) followed by anodic cleaning at 95 amp./ sq. ft., rinsing in tap water and then in distilled water.

(6) The same as in (1), the pickling taking place while under the influence of ultrasonic vibrations.

Pins without smut were treated by procedures (1), (2), and (6). Then they all were electroplated with zinc in purified baths. In all cases the cathode current density was 18 amp/sq. ft. for the first three minutes and then 38 amp/sq. ft. for the last 12 minutes. After plating (but without baking) the pins were subjected to the slow bend test already described. The results of the bend tests are given in Table V, below:

.(2 percent) and hydrochloric acid (0.5 percent) they become even more desirable in appearance than those obtained from baths containing brighteners. The hydrochloric acid can be replaced by the same proportion of sulphuric acid but the resulting appearance is not quite as desirable. The amount of zinc removed during this acid treatment is negligible, no visible hydrogen being liberated.

It has been found that, to give satisfactory results with respect to hydrogen embrittlement prevention, stable baths should be used whose proportions of ingredients satisfy the formula (OH- normality)-(CN normality):l.2, and also preferably that the ratio of CN- normality/Zn++ normality: 1.2.

TABLE V Results of slow bend tests on zinc plated pins treated by various procedures prior to plating Bend test results On pins without smut On pins with smut Degrees of bend Degrees of bend before breaking Percent before breaking Percent of pins of pins Surface preparation prior to plating Min Max. passed Min Max. passed (1) Vapor degreased, pickled rinsed 90 90 (2) Vapor degreased, pickled, anodically cleaned, rinsed 49 90 (3) Vapor degreased, wet-blasted, scrubbed, rinsed 33 71 (4) Vapor degreased, wet-blasted, scrubbed, rinsed, pickled, rmsed 60 90 (6) Vapior degreased, wet-blasted, scrubbed, rinsed, anodieally cleaned, 61 61 rinse (6) Vapor degreased, pickled while under the influence of ultrasonic vibrations, rinsed TABLE VI Bath composition Ounces/gallon ON-N Ratio Test No. NaOn NaOH n Zn++N It has been found that the bath of Test 21 is superior to all of the other baths because of the higher current efficiency obtainable in it (approaching 100 percent under the conditions used). For instance, plating under identical conditions, a zinc coating 0.0006 in. thick can be obtained from the bath of Test 21 in the same length of time as a coating 0.0004 in. thick from the bath of Test 17.

The grain structure of zinc coatings from the bath of Test 21 is at least as fine-grained as that from the other baths investigated as long as the latter contain no addition agents. When the coatings from the bath of Test 21 are dipped for one minute in a solution containing nitric acid Data Illustrating It was also found that pins which had been zinc plated in the baths whose analyses fell on or very near to a straight line, of the formula (OH- normality).(CN- normality)=1.2 gave a satisfactory performance in the bend test. This showed that they had not been rendered unserviceable by hydrogen embrittlement. Stable baths could be used for considerable periods of time for plating pins which performed satisfactorily in the bend test. 011 the other hand, pins plated from unstable baths whose analyses had moved away from the straight line, no longer performed satisfactorily in the bend test.

Baths having CN- normality Zn normality ratios of 1.2, but which have higher CN- and OH- concentrations than the baths shown in Table VI above, have been found to produce crystalline deposits of zinc which are undesirable in appearance. Baths having CN- normality Zn++ normality ratios of 1.2 which have lower concentrations of CN" and OH- than those of the baths shown in Table VI above have been found to be unstable and pins plated in them fail in the bend test.

It has been found, in accordance with aspects of this invention, that when the cathode current density is adequately controlled, the cathode surface preparation (i.e. the preparation of the steel surface) and the bath analysis are also essential. The data supporting these conclusions is set out in Table VII below.

TABLE v11 the importance of controlling cathode surface preparation and bath analysis in addition to the cathode current density [Current density at 18 amp/sq. ft. for 3 min. and 38 amp/sq. it. for 12 min.]

Percent of Bath analysis (oz./ga.) pins passed Surface of in slow bend pins Surface preparation NaCN NaOH 11 test No smut ..{HC1 pickle 11. 5 14. 6 5.0 0 o 11.6 15. 6 5. 3

Smut HCl pickle 11.8 16.0 6. 2 0 H01 pickle with ultrasonic. 11.8 16.0 5. 2 100 HCI pickle 9. 5 14. 3 4. 1 0

H01 pickle with ultrasonic 9. 5 14. 3 4.1 10 0 It is seen from the above table that a simple hydro chloric acid pickle gives satisfactory results when Type 1062 steel without smut is used. When smut is present, it is necessary to use ultrasonic treatment in conjunction with the hydrochloric acid pickle. It is preferredto use ultrasonic treatment in any case.

It is noted, therefore, that the important conditions which should exist at the surface of the cathode, according to another aspect of this invention, during plating to maintain freedom from hydrogen embrittlement appear to be: firstly, an adequately clean metallic surface upon which the zinc is to be deposited; secondly, a properly formulated and purified plating bath containing zinc ions; thirdly a preliminary current density which will give as high a current eificiency as possible nad consequently a minimum of hydrogen evolution; and finally, a final current density which will give the required zinc coating thickness in the minimum time.

It is also observed that the surface of the anode should be free from non-metallic particles. Anodes operating in purified plating baths of the composition shown in Table VI are substantially free from such materials.

There are combinations of CN" and OH"- in the baths which give the greatest stability and thus the best plating results. The baths having these combinations have a composition satisfying the equation (OH normality) -(CN- normality)=1.2. The Zn++ normality is also important because the deposition of zinc at the cathode will be more diflicult if the concentration of zinc in the bath becomes too low.

Two types of impurities in the zinc bath should be avoided. The first is carbonate. One method of removing carbonate from the bath is by a zinc dust electrolytic treatment. It is preferred, however, to use stable baths such as listed in Table VI above.

The second type of impurity comprises even small amounts of ions of metals such as nickel and iron. The zinc plated from such a bath contains traces of these other metals and thus is less resistant to certain types of corrosion. Pins of high strength steel Type 1062 fail in the slow bend test when zinc plated in a bath containing traces of such other metals. These ions also can be removed from the bath by the zinc dust electrolytic treatment.

The maintenance of a plating bath free of impurities such as carbonate and the ions of nickel, iron and similar metals, is imperative if the resulting zinc plated high strength steel is to be substantially free of hydrogen embrittlement. One very efiicient method of purifying the bath is the zinc dust electrolytic treatment. This purification may take place in batches or it may be continuous. The latter method is recommended, particaularly for comparatively large installations.

As a result of the test described above, it has been found, according to a further aspect of this invention, that:

(1) Zinc plated high strength steel, substantially free from hydrogen embrittlement as indicated by the slow bend test, can be produced without including baking in the process;

(2) The operation of this procedure preferably requires proper control of the cathode surface preparation, plating bath analysis and cathode current density;

(3) Hydrochloric acid pickling in conjunction with ultrasonic vibration is a very suitable preparation for the steel cathode surface;

(4) Purified plating baths which are satisfactory from the standpoints of stability and freedom of the steel products from hydrogen embrittlement have compositions satisfying the formula (OH'" normality)(CN normality) 1.2;

(5) Unpurified baths which are unsatisfactory from the standpoint of stability but which are temporarily satisfactory from the standpoint of freedom of the steel prod- 16 ucts from hydrogen embrittlement, also may have the same composition;

(6) One satisfactory bath has an analysis of 11.8 oz./gal. (89 g./l.) NaCN, 15.6 oz./gal. (117 g./l.) NaOH and 5.3 oz./gal. (40 g./l.) Zn (Bath 21);

(7) The zinc dust electrolytic treatment is a very satisfactory process for purifying these baths;

(8) An adequate cathode current density schedule may be 18 amp./sq.ft. for three minutes followed by 38 amp./ sq.ft. for 12 minutes. The current density can be as high as amp./ sq. ft. withan accompanying reduction in the plating time.

(B) CADMIUM PLATING BATHS Cadmium plating was done on steel cathodes some of which were cylindrical pins of Type 1062 material, while the remainder were of cold-rolled sheet of Type 1010 material. Type 1010 steel was tested for adherence of coating. The pins were 0.4 cm. (0.15 in.) in diameter and varied in length between 5.7 and 8.3 cm. (2.25 and 3.25 in.). They had been produced from drawn wire and had been austempered in the bainitic range resulting in a Rockwell C hardness of 52-56. When received in this laboratory they were covered with a thin uniform blue coating of iron oxide. The sheet cathodes were 5.1 cm. (2.0 in.) square and 0.08 cm. (0.03 in.) thick and were comparatively free from visible oxide.

Prior to cadmium plating, the surfaces of all cathodes were treated as follows:

(1) Degreasing with suitably stabilized trichlorethylene in a Model DLLV-B laboratory vapor degreaser;

(2) Pickling at room temperature in 18 percent (by weight) hydrochloric acid produced by diluting the concentrated (37 percent) reagent acid with distilled water. The operation was performed in a glass container which had been placed between two ultrasonic transducers containing Sonogen Z elements operating at a frequency of 25 kc. -3 percent and a load of -140 ma. The transducers were submerged in water 15 cm. (6 in.) apart. The pickling time was one minute, too long a time giving inferior results.

(3) Through rinsing in water and introduction into the plating bath with as little delay as possible.

The electroplating was performed in two litres of bath in a rectangular glass container 12.7 cm. (5.0 in.) wide by 19 cm. (7.5 in.) long by 12.7 cm. (5.0 in.) deep. A cadmium anode, cut from a commercial anode, was placed against each of the two narrow sides of the container, the immersed portion of each anode being 5 by 5 cm. (1.9 by 1.9 in.). The cathode was centrally located between the two anodes at least 1.3 cm. (0.5 in.) below the surface of the bath. The latter was maintained at 25:2 C. (77 F.) and was agitated slowly by a rotating Teflon-coated steel propeller. The direct current electricity was supplied from a copper/copper oxide rectifier suitably equipped with controls and meters.

The plating baths were prepared from sodium cyanide (NaCN) sodium hydroxide (NaOH), and cadmium oxide (CdO) of reagent quality, distilled water being the solvent. They were filtered through Whatman No. 52 filter paper to remove foreign particles when required. Additional distilled water was used to compensate for losses due to evaporation. Each bath was analyzed frequently to detect changes in composition.

Coating thickness testing The thicknesses of the electroplated cadmium coatings were determined by means of the Aminco-Brenner Magne Gage and by a stripping method. The latter consisted of Weighing the plated sample (W on an analytical balance and then immersing it in 29 percent (by weight) hydrochloric acid. It was left in the acid until the coating was completely stripped (as indicated by a sudden reduction in hydrogen evoluation) after which it was thoroughly rinsed and rapidly dried in a hot air stream. It was allowed to cool to room temperature and then reweighed (W The coating thickness was calculated from the coating weight The first tests carried out were to determine the stability of the cadmium plating baths.

' The procedure used was as follows. Each one was elect'rolyzed at 25 C. (77 F.) using cadmium anodes, square sheet or pin cathodes and a cathode current density of 4.1 amp/dm. (38 amp/ft. A sample of each bath was removed periodically for analysis for sodium cyanide, sodium hydroxide, cadmium and carbonate and, at the same time, the cadmium coated cathode was replaced by a new one. This was repeated a number of times, Analytical results showing that the compositions of these baths changed It has also been found that the plating cell voltage is unstable in the case of baths according to the prior art such as the baths of Tests Nos. 23, 24, 25 and 26, but is steady in the case of the baths for use in accordance with this other aspect of this invention (the baths of Tests Nos. 27, 28 and 29). The unstable voltages in the case of the former baths probably are due to the dark non-metallic =films that are formed on the surfaces of the anodes during plating, as indicated in the table. These films would be expected to increase the voltage drop and decrease the efliciency of metal solution at the anodes. In the case of the baths for use in accordance with this other aspect of this invention, (the baths of Tests Nos. 27, 28 and 29) such compounds are not formed at the anodes so that the materially during the plating, i.e., that the baths had poor efficiency of the anode process would be greater. stability, are given in Table VII below. The fact that there was more gassing at the cathodes TABLE VH1 tvirlhfm tletlbaths 2f TefitsbNofi. 2%124, 2;Iand2276 izvgre ellezcgoyze anwente atso ests 0s. an i cl tiriii 5 515332? rlifi/iilifi were treated in that way, indicates that the efliciency of Length cadmium deposition is greater in the case of the newly T? of Nature of iri g li Bath analysis (g'lL) Bath i bathsf th d d d int e appearance 0 e ca miurn coatings pro uce in NO- cthodes (mm') NaCN NaOH Cd Stab y the baths of Tests Nos. 28 and 29 was slightly more atsheet 3 53;? ii? 3 1:; }P0Or' tractive than that of the coatings produced in the bath of 780 72.8 9.0 29.3 25 Test No. 27, and considerably more attractive than in the 24 Pins g 58:8 Do. case of the baths of the prior art.

0 167.3 36.8 52. 5 Although no actual throwing power measurements were 25 sheet 338 Egg it; i made, it was obvious from the inspection of the various 0 130.0 88.5 108.0 cathodes that the covering abilities of the baths of Tests 26 $53 i221; 3312 18233 Do 30 8- 7, 28 and 29 were superior to those of the baths of 27 g 27 if? Tests NOS. 23, 24, 25 and 26.

0 5 'In a few experiments with baths in which the OH nor- "i 240 34.5 mality was greater than the CN- normality, it wa fo d 29 d0 8 2 232 that the cell voltage tended to decrease on electrolysis and a dark non-metallic film was formed on the anode. The cadmium coatings produced were as attractive looking as sta esman: 233;? eaaisers: 28 v a ecrys as were arger. e covering a 11 was ess stable. These stable baths thus satisfy the relationship. than that 0f the baths of es s NOS- 27 28 mi! 29, but onn0rmality =CN normallty superior to that of the baths of Tests Nos. 23, 24, 2s 1 d 26. As illustrated in Table VIII, baths having an OH- noran r'nality greater than their CN normality also are unstable. In the bath of Tes't 29 coatlng 1110101688 0f 0-0003 It has also been found that a bath which was 2.00 normal W Produced one t a a cathode current in'NaON, 2.25 normal in NaOH and 0.63 normal in Cd denslty of P{ (180 amp/fL It IS. important to begin with, i.e. a bath having an OH- normality greater to note that coohng lf mlght be f q 1n the t its normality is unstable bath when using such a high current density.

Consequently, for cadmium plating according to another It has been found that 8 the hs 013' this other aspect of this invention, it is preferred to use a stable aspect of the F 9 mventlon t ficlency 0f 99 cadmium plating bath in which the OH normality=CN- Percent was Obtamed at a current denslty of P normality, and preferably also one in which the ratio of dm. (38 amp/ft?) and a current efiiciency of 88 percent CN normality/Cd++ normalit is about 2.8. Was Obtained at a current density of 12.9 amp/dm. (120 Baths representative of the prior art are unstable even amp/ft. at a current density of 4.1 amp/dm. (38 amp/ftF). On The above characteristics are summarized in Table IX the other hand, 'baths for use according to this other aspect below:

" TABLE IX Characteristics of typical cadmium plating baths during electrolysis [Cathode current density=4.1 amp/timfi] Bath of Test No. Baths of Test Nos. Bath of Test No. Bath of Test N 0. Baths of Test Nos. 23 24 and 25 26 27 28 and 29 C ll oltage r; Unstable Unstable Unstable Steady Steady, Anode appearance--. Very dark ry dark V ry dark Bright metallic Bright metallic. Cathode gassing Slight at first and Slight t first nd Considerable None visible None visible.

slowly increasing. rapidly increasing. Cadmium coating appearance. Slightly grey; large Metallic white; Metallic white; large Slightly grey; tiny Metallic white; crystals. crystals tiny at crystals. crystals. tiny crystals. first then larger. Covering ability Fail at fi OO 1! Good Good.

becoming poorer.

of this invention are' stable at much higher current densities. The bath of Test No. 27, which is comparatively dilute, is stable at 8.2 amp/dm. (76 amp/ft?) but is unstable at 10.7 amp/dm. (100 amp/ft. The baths of Tests Nos. 28 and 29, which are more highly concentrated, are stable at current densities as high as 19.4 amp/drn. (180 amp/ft?) and perhaps higher. These particular experiments were done with pins as cathodes.

Additional tests were conducted in the presence of brighteners.

(1) A standard brightener was added to the bath of Test No .28 and a number of pins were plated at 12.9 amp/dm. amp/ft The plated pins were very bright in appearance. Above this current density the results were less attractive.

(2) Pins which have been plated at 12.9 amp/dm. (120 amp/ft?) in the bath of Test No. 28 (not containing brightener) were immersed in a solution containing 2.5 percent of nitric acid and 0.5 percent of hydrochloric acid (by weight) for two minutes and then thoroughly rinsed and dried. The surfaces of the pins were quite bright and attractive in appearance. The loss of metal was negligible.

As a result of these tests, it can be said that:

(1) The cadmium cyanide electroplating baths of the prior art (which have OH- normalities less than their CN- normalities):

(a) Change in composition during plating and their instabilities increase greatly as the cathode current density is increased;

(b) Have a metal solution efiiciency at the anodes of well below 100 percent as shown by the formation of a non-metallic film on the anode surfaces; and

Produce on the cathodes comparatively coarse crystalline cadmium deposits, which have a comparatively poor covering ability.

(2) Cadmium cyanide baths whose OH normalities are greater than their CN- normalities, are unstable.

It is also observed that, using cadmium baths according to this other aspect of this invention, i.e. cadmium cyanide baths whose OH-normalities are equal to the CN- normalities (a) Are stable under normal plating conditions, and some of them are stable at cathode current densities much higher than normal;

(b) Have higher cathode current efliciencies for a given cathode current density than is the case with baths having OH normalities less than their CN- normalities;

(c) Have a considerably improved metal solution efficiency at the anodes as shown by the absence of nonmetallic films on the surfaces, and by the maintenance of a constant cadmium content in the bath;

(d) Produce much finer crystalline deposits of cadmium on the cathodes, which have a much improved covering ability; and

(e) Produce deposits that can be brightened satisfactorily by a dilute acid treatment, or produce bright deposits when they contain suitable additives.

Particularly preferred baths are those having the characteristics shown in Table X.

Further tests were carried out using the pretreating technique of one aspect of this invention with the stable zinc cyanide baths, described above according to another aspect of this invention, and with the cadmium cyanide baths, described above according to a further aspect of this invention.

Notched test bars of Type E4340 steel prepared as described hereinabove were subjected to the nitric acidacetic acid cleaning treatment described hereinabove which did not produce embrittlement. They then were plated with cadmium as described hereinabove using bath A of composition shown in Table X. A cathode current density of 8.1 amp/dm. (75 amp/ft?) was used and the bars were not baked after plating. They were then tested in the Satic creep and stress rupture machine at a constant load of 249K p.s.i. (75 percent of the average UTS). Four bars which had received the nitric acid-acetic acid treatment were removed unbroken from the test after scribed hereinabove which did not produce embrittlement. They were then plated with cadmium as described hereinabove using A bath composition shown in Table X. A cathode current density of 8.1 amp/(1m? (75 amp/ft?) was used and the rings were not baked after plating. The cadmium plated rings were stressed at 240K p.s.i. for 672 and 1056 hr. before they failed. A minimum of 250 hr. under the above stressed conditions is usually required by the aircraft industry.

These results obtained with high strength steel Type E4340 notched test bars and Douglas stress rupture rings further substantiate those obtained with Type 1062 and Type 4037 steel pins.

The cadmium electroplating experiments which will now be described in greater detail were performed in the plating barrel already referred to. Each charge in the barrel consisted of 1240 pins, or 12-40 pins together with 1 or 2 tensile test bars.

In every plating experiment, the cadmium film was built up to 1 mil or more in thickness, since experience has taught that no additional embrittlement would occur at thicknesses greater than that value. In some cases, the film was built up to about 8 mils so that the quality of the adherence of the cadmium to the steel could be determined.

The results of these tests are shown below in Table XII.

TABLE XII Effect of pretreating followed by plating in a stable cadmium plating bath, on the embrittlement of Type 1062 and 4037 steel pins Degree of embrittlement at rinse, HNO CH CO OH (13) (5 min), rinse, ultrasonic water rinse, plate.

1 Not sufficiently clean for plating.

It is seen from the above table that in Test 30 Type 1062. pins had increased in embrittlement when they had been plated after being subjected to the pretreatment used in Test 11, which had given excellent freedom from embrittlement but only good cleanliness and smoothness when used alone. On the other hand, it was shown in Test 31 that both types of steel had excellent freedom from embrittlement when they were plated after being subjected to the pretreatment used in Test 15, which had given excellent cleanliness and smoothness as well as excellen freedom from embrittlement when used alone. This would indicate that pretreated pins must be excellent with regard to cleanliness and smoothness as well as with regard to freedom from embrittlement if they are to be free from embrittlement after cadmium plating.

The pretreatmnt and plating procedure used in Test 32 was as effective as that in Test 31 and was simpler to perform. The excellence of the procedure used in Test 32 was further demonstrated when pins of both types of steel, which already had been bent through in the Hounsfield testing machine, were placed in a vise and bent almost through without breaking. It is of interest to note that the copper coat used in Test 32 could be eliminated without any ill effects when treating Type 1062 pins, but not when treating Type 4037 pins.

Testsv were also conducted to determine whether the procedure of one aspect of this invention would eliminate embrittlement already present in steel.

During these tests, pins of these two types of steel were severely embrittled by cathodic treatment in hydrochloric acid. The pins embrittled in the same way were given a copper coat and treated in HNO CH COOH (A) acid cleaning solution, which removed all significant embrittlement. Embrittled pins, after treatment in the above manner and then cadmium plated in a stable bath, still were not embrittled appreciably. These results are summarized in Table XII'I below.

TABLE XIII E limination of significant embrittlgemlent from pins of Type 1062 and 4037 s cc s It is interesting to note that the combination of copper coating and treatment in HNO CH COOH acid cleaning solution is useful in both the prevention and elimination of embrittlement from these steels.

In this patent application oz./ gal. refers to ounces (avoirdupois) per gallon (British) and can be converted to grams per liter by multiplying by 6.236.

We claim:

1. A process for plating high strength steel with a corrosion protective metal selected from the group consisting of Cd and Zn comprising the steps of (I) pretreating said high strength steel to provide a clean, smooth, substantially oxide free surface which is conducive to electroplating without causing significant hydrogen embrittlement which comprises the steps of:

(A) providing a surface of the steel smooth, clean and substantially oxide free; (B) then treating said surface with an acid solution selected from the group consisting of (i) nitric acid, about 10 to about 15 by weight, acetic acid, about 4 to about 35% by weight, balance substantially all water; and (ii) nitric acid, about 23% by weight, acetic acid, about 31-35% by weight, phosphoric acid, about 27-30% by Weight, balance substantially all water, thereby to maintain said surface smooth, clean and substantially free of oxide; and (C) finally removing smut from the steel surface after the acid treatment; and

(H) electroplating said corrosion protective metal coating on said pretreated steel from a cyanide bath of the ions of said protective metal, said Cd coating being electroplated from a cyanide bath in which the OH- normality equals the CN- normality, said Zn coating being electroplated from a cyanide bath in which the (OH- normality)(the CN- normality)=1.2.

2. A process for plating high strength steel with a corrosion protective metal as claimed in claim 1 wherein steps (I) (A), (B) and (C) comprise the following steps:

(a) cleaning away visible oxide film from said steel surface;

(b) producing a thin fugitive porous coating of copper,

or zinc on said treated steel;

() removing said fugitive metal coating and substantially simultaneously producing a smooth surface substantially free of minute sharp edges by treatment 22 with an acid solution selected from the group consisting of (i) nitric acid, about 10 to about 15% by weight, acetic acid, about 4 to about 35% by weight, balance substantially all water; and (ii) nitric acid, about 23% by weight, acetic acid, about 31-35% by weight, phosphoric acid, about 27-30% by weight, balance substantially all water; and

(d) removing smut from said steel surface after said treatment.

3. The process as claimed in claim 2 wherein:

step (a) comprises cleaning away visible oxide film from said steel surface with hydrochloric acid;

step (b) comprises depositing, electrolessly, a thin,

porous copper flash; and

step (d) comprises removing smut and cleaning said surface resulting from step (c) by contacting the surface with water.

4. The process of claim 3 wherein the plating bath is one in which the ratio of CN- normality/Zn++ normality ranges from 1.07 to 1.25.

5. The process of claim 4 wherein the bath has the analysis:

NaCN 44 NaOH 83 6. The process of claim 4 wherein the bath has the analysis G./l. NaCN 47 NaOH 88 Zn+ 22 7. The process of claim 4 wherein the bath has the analysis:

G./l. NaCN 71 NaOH 107 Zn++ 31 8. The process of claim 4 wherein the bath has the analysis:

one in which the ratio of CN- normality Cd++ normality is about 2.8.

10. The process of claim 9 wherein the bath has the following analysis:

G./l. NaCN 98.3 NaOH 80.3 Cd 34.5

11. The process of claim 9 wherein the bath has the following analysis:

G./l. NaCH 137.3 NaOH -1--. 112.5 Cd++ 48.8

12. The process of claim 9 wherein the bath has the following analysis:

G./l. NaCN 57.8

NaOH 44.3

13. The process of claim 3 wherein the copper flash is deposited from an aqueous solution of copper sulphate at a pH of 2.2-2.8.

14. The process of claim 13 wherein the copper sulphate is at a pH of 2.6.

15. The process of claim 3 wherein the copper flash is deposited from the acid cleaning solution containing nitric acid and acetic acid which has previously been used to dissolve the copper flash in step (c) 16. The process of claim 3 wherein the water used is water under sufficient hydraulic pressure to remove said smut.

17. The process of claim 3 wherein the water used is water under suflicient pneumatic pressure to remove said smut.

18. The process of claim 3 wherein the water used is rinse water in conjunction with ultrasonic vibration.

19. The process of claim 2 wherein step (b) comprises producing a thin porous copper coating on said treated steel surface.

References Cited UNITED STATES PATENTS 1,800,947 4/1931 Mason 20429 2,158,992 5/1939 Cook 156-18 2,109,675 3/ 1938 Miller 20434 2,472,786 6/1949 Bowerman 204-34 X 2,776,255 1/1957 Hammond et a1. 20434 X 2,915,444 12/1959 Meyer 204145 3,043,712 7/1962 Toomey 11750 3,066,084- 11/1962 Osterman et a1. 204141 X 3,096,182 7/1963 Berzins 106-1 3,205,086 9/ 1965 Brick et a1 l17--4 3,282,731 11/1966 Hudson et a1 1343 X 3,061,494 10/1962 Snyder et a1 25279.4 X

OTHER REFERENCES W. Dingley et al., Technical Proceedings, American Electroplaters Soc., vol. 51, pp. 6670, (1964).

W. Dingley et al., Technical Proceedings, American Electroplaters Soc., vol. 50, pp. 71-77, (1963).

R. 0. Hull et al., Proc. American Electroplaters Soc., vol. 38, pp. 141 (1951).

GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 

